Electrostatic Charging of Granular Materials

The effects of electrostatics can have a large impact on granular flows. These effects can cause segregation and agglomeration and have an important impact on many industrial processes. The charging of grains also leads to dangerous electric discharges and dust explosions. Many industries spend large sums of money to prevent or control electrostatic charging. The pharmaceutical industry is especially susceptible because it requires that small amounts of drug be mixed with large amounts of excipient and the unpredictable nature of electrostatics can cause segregation and product non-uniformities.

How charge is accumulated in flowing granular materials through a stainless steel cylinder was analyzed. It has been found that the net charge accumulated by the grains was dependent on the surface area of the cylinder initially covered by particles. In mixtures that were both homogeneous and heterogeneous, only those grains in contact with the walls of the cylinder became charged. The preferred charging of one component in a mixture of granular materials based on composition or size could result in segregation, producing a change in product concentration. In bi-dispersed mixtures, the smaller particles were observed to segregate toward the walls; only these smaller particles charged, even in a mixture of 50% larger particles by volume. These experiments show the possibility for the segregation of smaller particles due to electrostatic effects.